1. Filed of the Invention
The present invention relates generally to a suspension control system for an automotive vehicle for adapting vehicular suspension characteristics to vehicular driving conditions and thereby achieving a satisfactory level of vehicular riding comfort and driving stability. More specifically, the invention relates to a technology for detecting or discriminating a road roughness dependent vehicular body attitude change mode among various modes of vehicular body change in order to accurately perform road roughness adapted suspension control. Still more particularly, the invention relates to a technology for detecting the road roughness condition requiring a roughness adapted suspension control, by means of an acceleration sensor or a G sensor monitoring lateral acceleration exerted on the vehicular body.
2. Description of the Background Art
In the modern automotive technologies, it is one of the trendy and important technologies for controlling suspension systems for adapting suspension characteristics, i.e. stiffness or damping characteristics, depending upon a vehicle driving condition. In general, adjustment of the suspension characteristics is performed in order to achieve both of riding comfort and vehicular driving stability at a high level. For instance, in order to provide higher level riding comfort, softer suspension characteristics are generally preferred in the normal and steady vehicle traveling condition. On the other hand, in a particular driving condition, such as cornering, or an accelerating or a decelerating state, a higher level riding comfort may be achieved at harder suspension characteristics. Furthermore, in many of the latter cases, a harder suspension is also desired from the viewpoint of vehicular driving stability. Therefore, in order to ascertain harmonization of the vehicular riding comfort and driving stability, it is essential to detect or discriminate various vibration modes on the vehicular body resulting from various factors or vehicular driving parameters.
In case of vehicular body vibration caused by roughness on the road surface, Japanese Patent First (unexamined) Publication (Tokkai) Showa 60-151111 proposes technology for detecting a road roughness originated vehicular body vibration by means of a vehicular stroke sensor which monitors a relative distance between a vehicular body and a suspension member rotatably supporting a road wheel. The shown system is facilitated based on the fact that different modes of vehicular vertical vibrations may have different frequency ranges in vibration. This is true in some aspect. For instance, in case that the cause of vehicular body vibration is road shock input through the road wheel, the vibration frequency is relatively high. On the other hand, in case that the cause of vehicular body vibration is a vehicular body attitude change, such as rolling, pitching or so forth, originated by an external force exerted on the vehicular body, the vibration frequency is relatively low. Therefore, a certain accuracy level discrimination can be performed for detecting vibration modes of the vehicular body by checking the frequency range of vibration.
However, on some occasions, the prior proposed technology in a discriminating vibration mode of the vehicular body is not satisfactorily accurate. For instance, when the magnitude of road roughness is substantial to cause a vehicular attitude change, the instantaneous vehicular body behaviour becomes similar to that in the vehicular rolling or pitching. In case that the vehicular body attitude change is caused due to a roughness of the road surface, it is generally desirable to soften suspension characteristics for absorbing vibration energy and thereby minimize vehicular body vibration. On the other hand, in case that the vehicular body attitude change is caused due to lateral acceleration during cornering which results in vehicular rolling or to longitudinal acceleration during vehicular acceleration or deceleration which results in vehicular pitching, it is desired to provide a harder suspension in view of vehicular driving stability. Therefore, depending upon the modes of vibration, the direction to adjust the suspension characteristics becomes opposite. In this reason, it is quite important for accurately detecting mode of variation cause on the vehicular body in order to ascertain adaption of the suspension characteristics to the vehicular driving condition.
In addition, in case of the above-identified prior proposed suspension control system, a vertical stroke sensor has to be provided for each of the suspension systems, i.e. front-left, front-right, rear-left and rear-right suspension systems. This causes a higher system cost and complicated circuit construction to facilitate use in the limited space of the automotive vehicle.